Fix njk.

Runtime/rendering/box.mixin.gen.cs

-using System;
 using System.Collections.Generic;
 using Unity.UIWidgets.gestures;
 using Unity.UIWidgets.ui;

Runtime/rendering/proxy_box.mixin.njk

                 return this.child.getMinIntrinsicWidth(height);
             }
 
-            return 0.0;
+            return 0.0f;
         }
 
         protected override float computeMaxIntrinsicWidth(float height) {
 
-            return 0.0;
+            return 0.0f;
         }
 
         protected override float computeMinIntrinsicHeight(float width) {
 
-            return 0.0;
+            return 0.0f;
         }
 
         protected override float computeMaxIntrinsicHeight(float width) {
 
-            return 0.0;
+            return 0.0f;
         }
 
         protected override float? computeDistanceToActualBaseline(TextBaseline baseline) {